In general, thermosetting resin powders for surface coating contain a binder composed of a curable resin which is solid at ambient temperature and melts above 60.degree. C, and a latent curing agent which does not react with the solid thermosetting resin at ambient temperature, and which is also solid at ambient temperature with a melting point above 60.degree. C. If the resin or curing agent softens below 60.degree. C the mixture is difficult to pulverize, and the powder tends to block on storage.
It is known that solid polyglycidyl ethers of 2,2 bis(4-hydroxyphenyl) propane can be used as the binder component in coating powders, in combination with curing agents of the classes of amines, amides and polycarboxylic acid anhydrides. The use of epoxy resins as binder component in coating powders has the advantage, that during cure no volatile material is formed which could cause gas bells and blisters, resulting in imperfect films. However, coatings prepared from these powders have insufficient resistance to weathering conditions, in particular, to exposure to UV radiation, and are therefore in general not suitable for outdoor use.
It is also known that electrical insulators for outdoor use can be made by casting of a cycloaliphatic polyepoxide with a cycloaliphatic polycarboxylic acid anhydride as the curing agent, and curing the mixture by heat. Attractive cycloaliphatic polyepoxides for outdoor use are glycidyl esters of cycloaliphatic dicarboxylic acids such as hexahydrophthalic acid and bicyclo[ 2.2.1]heptane, 2,3-dicarboxylic acid. These esters are liquid at 25.degree. C, and can therefore not be used as major binder components in coating powders.
It has been proposed in U.S. Pat. No. 3,535,289 to make a soluble fusible resin for coating powder application by reacting the diglycidyl ester of hexahydrophthalic acid with an equimolar amount of hexahydrophthalic acid. The resin, although solid at ambient temperature, has a higher epoxy equivalent weight than desirable for a powder coating component. On the other hand, if the amount of hexahydrophthalic acid is reduced to prepare a resin with an epoxy equivalent weight in the desired range, the product is liquid at room temperature, or melts too low to be useful in a powder coating composition.
It is generally regarded by those in the coatings art that in order to obtain epoxy resin-based coatings suitable for outdoor use, the use of aromatic components should be avoided.
It has now been found that certain epoxy resin compositions which do contain specific aromatic components can be used as binders in powder coating compositions with very good applicability for outdoor use.